JPH08286106A - Condensing optical system for optical scanner - Google Patents

Abstract

PURPOSE: To suppress the number of parts, to prevent the increase of a cost, and also to accurately correct a plane tilt, as for a condensing optical system used for an optical scanner. CONSTITUTION: Constant speed scanning with laser beams L emitted from a laser beam source 11 and deflected by a polygon mirror 13 which is rotated at high speed is executed on a sensitive material 20 by an fθ lens 14 in a main scanning direction X, and also, the shift in a subscanning direction Y owing to the plane tilt of the polygon mirror 13 is corrected by the fθ lens 14 formed of a toric surface whose sectional form in a main scanning direction is aspherical and a cylindrical mirror 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a condensing optical system used in an optical scanning device which deflects a laser beam or the like to scan a predetermined surface.

[0002]

2. Description of the Related Art Conventionally, an optical scanning device for performing optical scanning with a light beam reflected and deflected by a rotary polygon mirror such as a polygon mirror or a galvano scanner is well known.

In this type of optical scanning device, the position of the scanning spot is changed in the scanning direction of the light beam (hereinafter referred to as the main scanning direction) within the scanning surface due to the fluctuation of the reflecting surface of the light beam in the rotating polygon mirror or the like. A phenomenon occurs in which the vertical direction (hereinafter referred to as the sub-scanning direction) fluctuates, which causes a problem that the pitch in the sub-scanning direction, that is, the interval between the scanning lines becomes an uncertain point. In the case of a rotary polygon mirror, the error in the parallelism with respect to the rotation axis of each reflecting surface due to manufacturing accuracy (called surface tilt), and in the case of a galvanometer mirror, the fluctuation during movement (called wobbling) is the cause. Hereinafter, these are collectively referred to as “troublesome”.

Therefore, in order to correct this surface tilt, a cylindrical lens or the like having a refractive power only in the sub-scanning direction is usually provided on the optical path between the deflecting device such as the rotating polygon mirror and the scanning image forming surface. A face tilt correction optical system is provided.

Further, the focal plane of the light beam deflected as described above has an arc shape, and when scanning on a flat surface, the spot diameter on the scanning surface of the light beam and the scanning speed are set during one scanning. fluctuate. Therefore, when the scanning surface is a flat surface, in order to prevent fluctuations in the spot diameter of the light beam and perform uniform scanning, f is placed on the optical path between the deflector and the scanning image forming surface.
It is well known to provide a θ lens or the like.

[0006]

By the way, as the f.theta. Lens, as disclosed in, for example, Japanese Patent Laid-Open No. 1-101510, an axisymmetric aspherical single lens is generally used. Such a single lens has the same refracting power in the main scanning direction of the light beam and the sub-scanning direction orthogonal thereto, and the refracting power realizes uniform scanning of the light beam in the main scanning direction. Since it was set to
With this single lens, surface tilt correction that requires a specific refracting power in the sub-scanning direction cannot be performed.For surface tilt, a surface tilt correction optical system such as a cylindrical lens or a cylindrical mirror is installed separately. It needs to be corrected.

However, in recent years, there has been a demand for higher precision and higher density scanning, and from this viewpoint, it is necessary to perform the face tilt correction with higher precision, and the face tilt correction is performed only with the cylindrical lens or the cylindrical mirror. Is becoming difficult to perform with high precision.

On the other hand, in the technique disclosed in, for example, Japanese Patent Laid-Open No. 64-35523, the scanning light condensing optical system has a cylindrical lens or toroidal lens as a surface tilt correction optical system and a toric surface as the single lens. This single lens is formed as an f.theta. Lens in the main scanning direction, and acts as a face tilt correction together with the cylindrical lens in the sub scanning direction. According to this technique, it is possible to perform the correction with a higher degree of accuracy than the correction of the surface tilt only with the cylindrical lens. However, in order to guide the light beam onto a predetermined scanning surface, it is usually necessary to provide a long mirror for changing the traveling direction of the light beam in addition to the above optical system in the design of the device.
With the above configuration, the number of parts is increased.

The present invention has been made in view of the above circumstances, and it is possible to suppress the number of parts to prevent the cost from increasing, and at the same time, it is possible to highly accurately perform the plane tilt correction. It is intended to provide.

[0010]

A condensing optical system for an optical scanning device of the present invention is a single lens for forming an image of deflected light on a predetermined scanning surface and scanning the scanning surface at a constant speed. And a cylindrical mirror having a refractive power only in a direction perpendicular to the scanning direction for surface tilt correction, that is, in the sub-scanning direction, and at least one surface of the single lens is The single lens is formed by a toric surface so as to correct surface tilt together with the cylindrical mirror, and the main scanning cross-sectional shape of the toric surface is an aspherical shape.

Here, the main scanning cross section means a cross section of a plane formed by a locus of a line where the deflected light passes through a single lens.

Further, in the above-mentioned single lens, both main scanning cross-sectional shapes of both surfaces are aspherical, the deflection point side surface is a toric surface, and the main scanning cross-sectional shape is convex toward the deflection point side. It is desirable that the light-collecting optical system for the optical scanning device of the present invention is formed.
The focal length of this single lens is f, the radius of curvature on the optical axis in the main scanning cross section on the side of the deflection point of the single lens is RX ₁ , the axial upper surface distance from the deflection point to the single lens is d ₁ , and the on-axis of the single lens is on the axis. The thickness is d ₂ , and the aspherical shape on the side of the deflection point of the single lens is expressed by the following formula (1) z = ch ² / [1+ {1- (1 + K) c ² h ² } ^1/2 ] + a ₁ h ⁴ + a ₂ h ⁶ + a ₃ h ⁸ + a ₄ h ¹⁰ (1) where z: a perpendicular line drawn from the point on the aspherical surface at a height h from the optical axis to the tangent plane of the aspherical vertex (plane perpendicular to the optical axis) Length h; height from optical axis c; curvature at aspherical vertex (reciprocal of radius of curvature; = 1 / RX
_i ) K; conical constants a _{1 to} a ₄ ; aspherical coefficients of the 4th, 6th, 8th, and 10th orders, which satisfy the following expressions (2) to (5) It is more desirable to do.

0.3 ≦ d ₂ / d ₁ ≦ 0.7 (2) 0.4 f ≦ RX ₁ ≦ 2.5f (3) −3.0 × 10 / f ³ ≦ a ₁ ≦ −5.0 / f ³ (4) 7.0 × 10 / f ⁵ ≦ a ₂ ≦ 6.0 × 10 ² / f ⁵ (5) That is, when the index value d ₂ / d ₁ is less than the lower limit value of the equation (2), the field curvature increases in the positive direction (overcorrection), When the value exceeds the upper limit, it becomes large in the negative direction (insufficient correction), and the wall thickness of the single lens becomes too thick, which is not preferable because the entire apparatus becomes large.

When the index value RX ₁ is below the lower limit value of the equation (3), the fθ characteristic increases in the negative direction, and above the upper limit value, it increases in the positive direction.

Furthermore, when the index value a ₁ is lower than the lower limit value of the equation (4) or the index value a ₂ is lower than the lower limit value of the equation (5), both the field curvature and the fθ characteristic increase in the positive direction. If the upper limit is exceeded, the curvature of field and the fθ characteristic will both increase in the negative direction, making correction difficult.

In the main scanning section on the side of the deflection point of the single lens, "convex toward the side of the deflection point" means convex on the optical axis (apex of aspherical surface). Therefore, even if the position deviated from the optical axis is partially concave toward the deflection point side, it may be convex on the optical axis.

Further, the single lens and the cylindrical mirror may be made of a plastic material other than a glass material which is usually used.

The deflection angle of the light may be set to a value of 60 ° or more.

[0019]

According to the condensing optical system for an optical scanning device of the present invention, a light beam repeatedly deflected in a fixed direction (main scanning direction) by a rotating polygon mirror or a galvano scanner of the optical scanning device is fθ. An image is formed on a predetermined scanning surface by a single lens such as a lens, and is converted so that main scanning can be performed at a constant speed on the scanning surface. Further, the light beam after passing through this single lens is incident on a cylindrical mirror before reaching the scanning surface, the traveling direction of the light beam is changed by this mirror, and the light beam is guided onto the predetermined scanning surface. Since the mirror has a refractive power only in the sub-scanning direction, it has no effect in the main scanning direction and merely reflects the light beam.

On the other hand, the scanning position of the light beam deflected by the deflector such as the rotary polygon mirror may change in the sub-scanning direction due to the tilt of the deflector. The light beam that fluctuates in the sub-scanning direction is incident on the single lens, but the fluctuation in the sub-scanning direction is corrected by the surface tilt correction effect of the toric surface. Further, the light beam emitted from this single lens is incident on a cylindrical mirror, which further corrects the surface tilt, and is reflected and converged on the predetermined scanning surface.

As described above, according to the condensing optical system for an optical scanning device of the present invention, at least one surface of the single lens is formed as a toric surface, and the main scanning sectional shape of the toric surface is an aspherical surface. With one lens, a good fθ characteristic can be obtained over a wide range, and in the sub-scanning direction, the single lens and the cylindrical mirror correct the surface tilt of the light beam to increase the number of constituent parts. It is possible to realize highly accurate face tilt correction while preventing it. Further, it is possible to prevent an increase in cost by preventing an increase in the number of parts, and it is also possible to prevent an accumulated error from increasing in an assembling process.

Further, good optical performance can be maintained even when the scanning angle (deflection angle) is 60 ° or more. On the other hand, by increasing the scanning angle in this way, the focal length is shortened, that is, the fθ lens. Since the distance between the image planes can be shortened, the entire optical system can be made compact, which is more preferable.

Further, in the above-mentioned single lens, the main scanning cross-sectional shapes of both surfaces thereof are both aspherical, the deflection point side surface is formed as a toric surface, and the main scanning cross-sectional shape of the deflection point side surface is set to the deflection point side. By forming it convex toward one side, it is possible to obtain a better fθ over a wide range with one lens.
The characteristics can be obtained.

Further, the focal length f of this single lens, the radius of curvature R ₁ of the side of the deflection point of the single lens in the main scanning section,
The axial surface distance d ₀ from the deflection point to the single lens, the axial thickness d _{1 of} the single lens, and the aspherical fourth and sixth aspherical surfaces represented by the equation (1) on the side of the deflection point of the single lens. Next aspherical coefficient a
_{When 1} and a ₂ are configured to satisfy the expressions (2) to (5), the field curvature and the fθ property can be appropriately corrected.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of a condensing optical system for an optical scanning device of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a perspective view showing a schematic structure of an optical scanning device using the focusing optical system of the present invention, and FIG. 2 is an excerpt of the focusing optical system of the present invention from the optical scanning device shown in FIG. 2A and 2B are views showing the configuration shown in FIG. 1A and FIG. 1B, respectively.

The optical scanning device shown in the figure has a laser light source 11 for emitting a laser beam L and a convex incident surface for forming a line image on the reflecting surface of a polygon mirror 13 which is an optical deflector. Cylindrical lens 12, a polygon mirror 13 for deflecting the laser beam L, which is rotated around an axis by a motor (not shown) and is incident on the reflecting surface in a predetermined direction, and the deflected laser beam L on a predetermined photosensitive material 20. The fθ lens 14 made of polymethylmethacrylate (PMMA) that forms an image on the fluctuating material 20 and causes the fluctuating material 20 to scan at a constant speed in the main scanning direction X, and the traveling direction of the laser beam L passing through the fθ lens 14 In order to correct the surface tilt of the polygon mirror 13 in the sub-scanning direction Y perpendicular to the main scanning direction X together with the fθ lens 14 And a cylindrical mirror 15 having a refracting power of at least one of the entrance surface and the exit surface of the fθ lens 14
Together with the cylindrical mirror 15 is formed of a toric surface so as to correct the above-mentioned surface tilt. The toric surface has an aspherical shape in its main scanning cross section (see FIG. 2A).

The sensitive material 20 as the medium to be scanned is transported in the direction of arrow Y (sub-scanning direction) by a transporting means (not shown).

In the focusing optical system of this embodiment shown in FIG. 2, the radius of curvature R [mm] of each lens surface of the fθ lens 14 and the reflecting surface of the cylindrical mirror 15 and the fθ lens from the reflecting surface (deflection point). Air spacing up to 14 incident planes, fθ
The center thickness of the lens 14, the air distance from the emitting surface of the fθ lens 14 to the reflecting surface of the cylindrical mirror 15, the air distance from the reflecting surface of the cylindrical mirror 15 to the scanning surface (hereinafter,
Table 1 shows a list of these ds (mm), which are collectively referred to as axial upper surface intervals, and the refractive index of the fθ lens 14 (d line; the same applies hereinafter).

[0030]

[Table 1]

In the column of the radius of curvature R in Table 1, the radii of curvature RX _{1 to} RX ₃ in the main scanning direction are cross sections (main scanning cross sections; main scanning cross sections; 2 (a)), which means the radius of curvature of the lens surface or the like, while the radius of curvature RY ₁ , RY ₂ , R
Y ₃ means the radius of curvature of the lens surface or the like in the section including the optical axis K in FIG. 1 and perpendicular to the main scanning section (see FIG. 2B). However, fθ lens
The radii of curvature RX ₁ and RX _{2 in} the main scanning direction of each surface of 14 indicate that they are aspherical surfaces, and the numbers marked with “*” indicate the radii of curvature on the optical axis (apex of aspherical surface). , The aspherical surface has a surface shape calculated by the following equation (1).

The sign of the numerical value is positive when it is convex on the deflector side and negative when it is convex on the scanning surface side.

Z = ch ² / [1+ {1- (1 + K) c ² h ² } ^1/2 ] + a ₁ h ⁴ + a ₂ h ⁶ + a ₃ h ⁸ + a ₄ h ¹⁰ (1) where z; optical axis From the point on the aspherical surface at height h from, the length of the perpendicular line drawn to the tangent plane of the aspherical vertex (plane perpendicular to the optical axis) h; Height from the optical axis c; Curvature at the aspherical vertex (Reciprocal of radius of curvature; = 1 / RX
_i ) K; conical constants a _{1 to} a ₄ ; fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients The focusing optical system of the embodiment shown in FIG. ) To (5) are further satisfied.

0.3 ≦ d ₂ / d ₁ ≦ 0.7 (2) 0.4 f ≦ RX ₁ ≦ 2.5f (3) −3.0 × 10 / f ³ ≦ a ₁ ≦ −5.0 / f ³ (4) 7.0 × 10 / f ⁵ ≦ a ₂ ≦ 6.0 × 10 ² / f ⁵ (5) where f represents the focal length [mm] of the fθ lens 14.

Next, the operation of the condensing optical system of this embodiment will be described.

A laser beam L is emitted from the laser light source 11, and the laser beam L is formed as a line image on the reflecting surface of the polygon mirror 13 by the cylindrical lens 12.

The laser beam L reflected by the reflecting surface of the polygon mirror 13 passes through the fθ lens 14, is reflected by the cylindrical mirror 15, is guided to the surface of the photosensitive material 20, and forms a scanning spot. To do. Here, since the polygon mirror 13 is rotated at a high speed in the arrow R direction, this scanning spot repeatedly performs main scanning on the surface of the photosensitive material 20 in the arrow X direction. During this period, the photosensitive material 20 is sub-scanned in the direction of the arrow Y, so that the scanning spot L scans the entire surface of the photosensitive material 20 by the main scanning and the sub-scanning.

More specifically, the laser beam L reflected by the reflecting surface of the polygon mirror 13 is main-scanned on the surface of the sensitive material 20 at a constant speed by the fθ lens 14, while the laser beam L is reflected by the reflecting surface of the polygon mirror 13. The scanning position changes in the sub-scanning direction due to the surface tilt. The laser beam L that fluctuates in the sub-scanning direction is partially corrected by the surface tilt correction function of the fθ lens 14 in the sub-scanning direction. Further, the laser beam L emitted from the fθ lens 14 enters a cylindrical mirror 15, and the mirror 15 further corrects the face tilt.

As described above, according to the condensing optical system for the optical scanning device of the present embodiment, the fθ lens 14 is formed by the toric surface, and the fθ lens 14 and the cylindrical mirror 15 cause the laser beam to be tilted part by part. By performing the correction, it is possible to realize the highly accurate face-down correction while preventing an increase in the number of component parts. Also, by preventing an increase in the number of parts, it is possible to prevent an increase in cost.
If the number of components further increases, the cumulative error increases in the assembly process. However, since the condensing optical system for the optical scanning device of the present embodiment has the same number of components as the conventional condensing optical system, Error does not increase.

3 (a) and 3 (b), the field curvature diagram and fθ by the converging optical system of this embodiment shown in Table 1 are shown.
The characteristics are shown respectively. From the graph shown in the figure, it can be seen that the optical system of the present embodiment is favorably corrected for aberrations.

Tables 2 to 9 below show other embodiments of the condensing optical system of the present invention. The basic configuration of these condensing optical systems is the same as that of the condensing optical system shown in FIG. That is, these condensing optical systems are similar to the condensing optical systems shown in Table 1 and FIG.
Regarding the θ lens 14, both main scanning cross-sectional shapes on both sides are aspherical shapes represented by the equation (1), and the deflection point side surface (incident surface of the light deflected by the polygon mirror 13).
Is a toric surface and its main scanning cross-sectional shape is formed to be convex toward the deflection point (polygon mirror 13) side.

Each of these condensing optical systems is also constructed so as to satisfy the above expressions (2) to (5). The symbols in each table have the same meanings as shown in FIG. 2 and Table 1.

Table 2 is a collection of the second embodiment in which the values of the respective elements defined by the conditional expressions (2) to (5) are set to have arbitrary values within the ranges indicated by the respective conditional expressions. 5 shows a list of the radius of curvature R [mm] of each lens surface of the fθ lens 14 and the reflecting surface of the cylindrical mirror 15, the axial upper surface distance d [mm], and the refractive index of the fθ lens 14 in the optical optical system.

[0044]

[Table 2]

FIG. 4 is a graph showing the field curvature characteristics (main scanning direction and sub scanning direction) and fθ characteristics of the condensing optical system of the second embodiment whose specifications are shown in Table 2.

From this graph, it is recognized that the focusing optical system of the present embodiment is favorably corrected for aberrations.

Table 3 is a collection of the third embodiment in which the values of the respective elements defined by the conditional expressions (2) to (5) are set to have arbitrary values within the ranges indicated by the respective conditional expressions. 5 shows a list of the radius of curvature R [mm] of each lens surface of the fθ lens 14 and the reflecting surface of the cylindrical mirror 15, the axial upper surface distance d [mm], and the refractive index of the fθ lens 14 in the optical optical system.

[0048]

[Table 3]

FIG. 5 is a graph showing the field curvature characteristics (main scanning direction and sub scanning direction) and fθ characteristics of the converging optical system of the third embodiment whose specifications are shown in Table 3.

From this graph, it is recognized that the focusing optical system of the present embodiment is favorably corrected for aberrations.

Table 4 shows the condensing optical system of the fourth embodiment in which the values of the elements defined by the conditional expression (5) are set to be substantially the upper limit values of the range represented by the conditional expression (5). , F
Lens surface of θ lens 14 and cylindrical mirror 15
Radius of curvature R [mm] of the reflecting surface of the
m] and the refractive index of the fθ lens 14 are shown.

[0052]

[Table 4]

FIG. 6 is a graph showing the field curvature characteristics (main scanning direction and sub scanning direction) and fθ characteristics of the condensing optical system of the fourth embodiment whose specifications are shown in Table 4.

From this graph, it can be seen that the focusing optical system of the present embodiment is favorably corrected for aberrations.

Table 5 is a collection of the fifth embodiment in which the values of the respective elements defined by the conditional expressions (2) to (4) are set so as to be substantially the upper limits of the ranges indicated by the respective conditional expressions. 5 shows a list of the radius of curvature R [mm] of each lens surface of the fθ lens 14 and the reflecting surface of the cylindrical mirror 15, the axial upper surface distance d [mm], and the refractive index of the fθ lens 14 in the optical optical system.

[0056]

[Table 5]

FIG. 7 is a graph showing the field curvature characteristics (main scanning direction and sub-scanning direction) and fθ characteristics of the converging optical system of the fifth embodiment whose specifications are shown in Table 5.

From this graph, it can be seen that the focusing optical system of this embodiment is favorably corrected for aberrations.

Table 6 shows the condensing optical system of the sixth embodiment in which the values of the elements defined by the conditional expression (2) are set to be approximately the lower limit values of the range represented by the conditional expression (2). , F
Lens surface of θ lens 14 and cylindrical mirror 15
Radius of curvature R [mm] of the reflecting surface of the
m] and the refractive index of the fθ lens 14 are shown.

[0060]

[Table 6]

FIG. 8 is a graph showing the field curvature characteristics (main scanning direction and sub scanning direction) and fθ characteristics of the converging optical system of the sixth embodiment whose specifications are shown in Table 6.

From this graph, it is recognized that the focusing optical system of the present embodiment is favorably corrected for aberrations.

Table 7 shows the condensing optical system of the seventh embodiment in which the values of the elements defined by the conditional expression (3) are set to be approximately the lower limit values of the range represented by the conditional expression (3). , F
Lens surface of θ lens 14 and cylindrical mirror 15
Radius of curvature R [mm] of the reflecting surface of the
m] and the refractive index of the fθ lens 14 are shown.

[0064]

[Table 7]

FIG. 9 is a graph showing the field curvature characteristics (main scanning direction and sub scanning direction) and fθ characteristics of the converging optical system of the seventh embodiment whose specifications are shown in Table 7.

From this graph, it can be seen that the focusing optical system of the present embodiment is favorably corrected for aberrations.

Table 8 shows the condensing optical system of the eighth embodiment in which the values of the elements defined by the conditional expression (5) are set to be approximately the lower limit values of the range represented by the conditional expression (5). , F
Lens surface of θ lens 14 and cylindrical mirror 15
Radius of curvature R [mm] of the reflecting surface of the
m] and the refractive index of the fθ lens 14 are shown.

[0068]

[Table 8]

FIG. 10 is a graph showing the field curvature characteristic (main scanning direction and sub scanning direction) and fθ characteristic of the condensing optical system of the eighth embodiment whose specifications are shown in Table 8.

From this graph, it can be seen that the focusing optical system of the present embodiment is favorably corrected for aberrations.

Table 9 shows the condensing optical system of the ninth embodiment in which the values of the elements defined by the conditional expression (4) are set to be approximately the lower limit values of the range represented by the conditional expression (4). , F
Lens surface of θ lens 14 and cylindrical mirror 15
Radius of curvature R [mm] of the reflecting surface of the
m] and the refractive index of the fθ lens 14 are shown.

[0072]

[Table 9]

FIG. 11 is a graph showing the field curvature characteristics (main scanning direction and sub scanning direction) and fθ characteristics of the condensing optical system of the ninth embodiment whose specifications are shown in Table 9.

From this graph, it can be seen that the focusing optical system of the present embodiment is favorably corrected for aberrations.

The aberration diagrams (FIGS. 4 to 11) corresponding to the condensing optical systems of the second to ninth embodiments, for simplicity, assume that the entry / exit of the reflecting surface of the polygon mirror 13 is not taken into consideration. The results are obtained, and therefore, the aberrations in the range where the scanning angle is negative are symmetrical with respect to the 0-degree line of each graph shown, and the illustration is omitted.

[Brief description of drawings]

FIG. 1 is a perspective view showing a schematic configuration of an optical scanning device using a condensing optical system of the present invention.

2A and 2B are configuration diagrams in which only the condensing optical system of the present invention is extracted from the optical scanning device shown in FIG. 1, in which FIG. 2A is an arrow A in FIG. 1, and FIG. 2B is an arrow in FIG. Show B respectively

FIG. 3 is an aberration diagram of the condensing optical system of the present embodiment,
(A) is a field curvature diagram and (b) is an fθ characteristic diagram.

4A and 4B are aberration diagrams of the condensing optical system according to the second embodiment, where FIG. 4A is a field curvature diagram and FIG. 4B is an fθ characteristic diagram.

5A and 5B are aberration diagrams of the converging optical system according to the third embodiment, where FIG. 5A is a field curvature diagram and FIG. 5B is an fθ characteristic diagram.

6A and 6B are aberration diagrams of the converging optical system according to the fourth embodiment, where FIG. 6A is a field curvature diagram and FIG. 6B is an fθ characteristic diagram.

7A and 7B are aberration diagrams of the converging optical system of the fifth embodiment, FIG. 7A is a field curvature diagram, and FIG. 7B is an fθ characteristic diagram.

8A and 8B are aberration diagrams of the converging optical system according to the sixth embodiment, where FIG. 8A is a field curvature diagram and FIG. 8B is an fθ characteristic diagram.

9A and 9B are aberration diagrams by the converging optical system of the seventh embodiment, FIG. 9A is a field curvature diagram, and FIG. 9B is an fθ characteristic diagram.

10A and 10B are aberration diagrams by the converging optical system of the eighth embodiment, FIG. 10A is a field curvature diagram, and FIG. 10B is an fθ characteristic diagram.

11A and 11B are aberration diagrams of the condensing optical system of the ninth embodiment, where FIG. 11A is a field curvature diagram and FIG. 11B is an fθ characteristic diagram.

[Explanation of symbols]

 11 Laser light source 12 Cylindrical lens 13 Polygon mirror 14 fθ lens 15 Cylindrical mirror 20 Sensitive material L Laser beam K Optical axis

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsu Noguchi 798 Miyadai, Kaisei-cho, Ashigarakami-gun, Kanagawa Fuji Photo Film Co., Ltd.

Claims (6)

[Claims] 1. A single lens for forming an image of the deflected light on a predetermined scanning surface and scanning the scanning surface at a constant speed, and only in a direction perpendicular to the scanning direction for surface tilt correction. A condensing optical system for an optical scanning device, which comprises a cylindrical mirror having a refractive power, wherein at least one surface of the single lens is formed by a toric surface so that the single lens corrects the surface tilt together with the cylindrical mirror. And a main scanning cross-sectional shape of the toric surface is an aspherical shape, a condensing optical system for an optical scanning device. 2. The main scanning cross-sectional shapes of both surfaces of the single lens are both aspherical, the deflection point side surface of the single lens is a toric surface, and the main scanning cross-sectional shape is toward the deflection point side. The condensing optical system for an optical scanning device according to claim 1, wherein the condensing optical system is formed in a convex shape. 3. The focal length of the single lens is f, the radius of curvature on the optical axis in the main scanning section on the side of the deflection point of the single lens is RX ₁ , and the axial upper surface distance from the deflection point to the _single lens is d. ₁ , the axial thickness of the _single lens is d ₂ , and the aspherical shape of the side surface of the deflection point of the single lens is expressed by the following formula (1) z = ch ² / [1+ {1- (1 + K) c ² h ² } ^1/2 ] + a ₁ h ⁴ + a ₂ h ⁶ + a ₃ h ⁸ + a ₄ h ¹⁰ (1) where z; from the point on the aspherical surface at a height h from the optical axis, the tangent plane ( The length of the perpendicular line drawn down to the plane perpendicular to the optical axis h; the height from the optical axis c; the curvature at the aspherical vertex (the reciprocal of the radius of curvature; = 1 / RX)
_i ) K; conical constants a _{1 to} a ₄ ; fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients, which satisfy the following expressions (2) to (5) The condensing optical system for an optical scanning device according to claim 2. 0.3 ≤ d ₂ / d ₁ ≤ 0.7 (2) 0.4 f ≤ RX ₁ ≤ 2.5f (3) -3.0 x 10 / f ³ ≤ a ₁ ≤-5.0 / f ³ (4) 7.0 x 10 / f ⁵ ≤ a ₂ ≤ 6.0 x 10 ² / f ⁵ (5) 4. The condensing optical system for an optical scanning device according to claim 1, wherein the single lens is made of a plastic material. 5. The condensing optical system for an optical scanning device according to claim 1, wherein the cylindrical mirror is made of a plastic material. 6. The condensing optical system for an optical scanning device according to claim 1, wherein a deflection angle of the light is 60 ° or more.